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Rear Wheel Slide


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#1 RogerThat

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Posted 27 July 2012 - 06:25 PM

Recently due to some setup changes and more aggressive braking, my rear wheel has gotten really light under braking (I am using front brake ONLY, no rear brake), and a couple of times has started to "come around". In both cases when I felt it start to get sideways I eased off the brake, it straightened out and then I turned the bike.

Here is my question - what would happen if I intitiated my steering action while the rear wheel was still a bit sideways? I see racers "backing it in" all the time on TV which indicates to me that I could probably turn it in with no ill effects - as long as the rear wheel was displaced to the inside of the turn. But what if the rear was stepped out to the outside, could I still turn without that creating a problem? It seems like it could lowside.

Follow-up question, what would happen if the rear wheel was all the way off the ground and I tried to turn the bike? Assuming I didn't overload the front wheel and make it slide (a daring assumption, I know), would the bike still turn ok or would it be too unstable from having the rear wheel in the air?

I have already improved my setup and hopefully eliminated the problem (and I know that staying relaxed on the bars helps keep the back end from wagging around) but the experience of feeling the back end get light and sideways made me curious about what would happen if I DID try to turn it while that was happening. Anybody tried it? :)

#2 Lnewqban

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Posted 28 July 2012 - 03:03 AM

Maybe something like this?: :)

http://www.youtube.c...1&v=fxL5S22_tMw

Joke aside, I believe that the response to your questions is all about the dynamics of the center of gravity or mass of the bike.

This CG is located between both wheels and somewhere under the seat.

For the first question:
As soon as the rear tire starts lifting up, the inertia of the CG will force it to rotate around the front tire's contact patch (because it is braking or slowing down).
Hence, that won't happen only if the CG is in line with that contact patch.
Imaging a rod (the bike while braking hard) being balanced vertically on the palm of your hand (the front tire patch).

As long as the CG of the rod is in vertical alignment with the point of support, the rod will not fall.
If the rear tire has traction, it is like you are holding the top of the rod with your other hand.

For the second question:
Also consider that the contact patch of the front wheel is not on the axis of the steering, due to the caster.
Because of that, any steering input will move that contact patch respect to the CG (to gain or to loose stability).

#3 mugget

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Posted 29 July 2012 - 08:54 AM

Dang!!! I can't believe that guy put his hand down! Posted Image Talk about survival reactions and incorrect response...

Anyway, Lnewqban explained it pretty well. To take it further and apply it to some riding situations I would add this:

Steering when the rear wheel is sideways: A potentially very risky proposition I would say! Yes you can probably get away with a small slide okay, but as the slide is larger, the stakes are higher. A potential downside comes to mind - what happens if the rear tyre suddenly regains grip? That is a recipe for a highside. So the whole manoeuvre depends on the rear tyre sliding in a predictable manner and gradually falling back into line behind the front wheel at some point during the turn. Also note that many times this does not go to plan, even for racers. (Actually, you can have the rear wheel sliding through the whole turn, but then it's not so much "cornering" as "drifting". And I have only ever seen stunt riders do that... Check the ICON car Vs. bike drift videos for reference.)

One AMA Superbike (I think) race in particular comes to mind, it could have been in 2010 or 2011 & I'm not sure which track... a rider was entering a turn, backing it in with spectacular style but then it all went wrong & he crashed out... the interesting thing is that the rider following him was riding wheels in line - and was gaining on him! There have been other threads on this forum talking about backing it in, what it comes down to is the fact that it's not the fastest way around a track. Great to watch, but not fast.

The other thing to consider when you see racers backing it in (Moto2, etc.) is the question of whether or not they actually prefer it to back in... I think many times it's simply due to reduced tyre grip. It seems like they would actually prefer to ride wheels in line, but it's often as the race progresses that more people start sliding the rear. One other thing which may may be especially relevant to those hard-riding Moto2 racers is the fact that if your "quick turn" ability is developed to a very high level, you can actually steer so fast that the rear wheel will break traction.

Rear wheel off the ground when steering: Many variables in this situation as well. I imagine that the highside danger is still there if you start to turn and the rear wheel touches down too far out of line with the front... Also - doesn't counter steering require that both wheels be on the ground? If the rear wheel is in the air and you push on the inside handlebar, won't the wheel point to the outside of the turn, and the bike follow it?


If you have a rear tyre to burn and are willing to try some things, you could practice some rear wheel lockups and see how steering inputs affect the bikes behaviour. Try this at your own risk... If you have access to a dirt bike that could be a great way to get an understanding of how a sliding rear wheel can affect steering inputs.

#4 Johnny Rod

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Posted 01 August 2012 - 02:21 PM

As said in another post recently, if you're turning while braking so hard the back wheel is more or less weightless, you're asking a lot of the front tyre. You can probably guess what happens next!

#5 mugget

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Posted 06 August 2012 - 06:54 AM

I found the video I was thinking of in another thread. From memory this rider was backing into this turn for the whole race prior to this incident... it just goes to show that even the professionals sometimes get it wrong. Just posting this so you can be aware of the potential consequences and make your decisions accordingly...






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